Are we really masters of our actions or, on the contrary, are we conditioned by a biological determinism? These doubts have been widely debated throughout the centuries of philosophy and psychology, and Libet’s experiment has contributed to intensify them.

Throughout this article we will talk about the experiment carried out by neurologist Benjamin Libet, as well as his procedures, results and reflections, and the controversy surrounding this study.

Who was Benjamin Libet?

Born in the United States in 1916, Benjamin Libet became a renowned neurologist whose early work focused on the investigation of synaptic and post-synaptic responses, and then focused on the study of neural activity and threshold sensations of these (i.e., the point at which the intensity of a stimulus generates a conscious sensation of change).

His first relevant research was aimed at establishing the amount of activation that certain specific brain areas need to release artificial somatic perceptions. As a result of these works, Libet began his famous research on people’s consciousness, as well as his experiments that related neurobiology and freedom .

As a result of his studies and reflections on freedom, free will and consciousness, Libet became a pioneer and a celebrity within the world of neurophysiology and philosophy. In spite of all these, his conclusions have not been free of criticism from researchers in both disciplines.

Libet’s experiment

Before Libet started his well-known experiments, other researchers such as Hans Helmut Kornhuber and Lüder Deecke already coined the term “bereitschaftspotential”, which in our language could be translated as “preparation potential” or “disposition potential”.

This term refers to a dimension that quantifies the activity of the motor cortex and the supplemental motor area of the brain as they prepare for voluntary muscle activity. That is, refers to the brain’s activity when planning to perform a voluntary movement . From this, Libet constructed an experiment in which a relationship was sought between the subjective freedom we believe to have when initiating a voluntary movement and neuroscience.

In the experiment, each of the participants was placed in front of a kind of clock which was programmed to make a full turn of the hand in 2.56 seconds. He was then asked to think of a point on the circumference of the clock chosen at random (always the same one) and at the moments when the hand passed by, he had to make a movement of the wrist and, at the same time, remember at what point on the clock the hand was at the moment of having the conscious sensation of going to make that movement.

Libet and his team named this subjective variable as V, referring to the person’s willingness to move. The second variable was coined as variable M, associated with the real instant in which the participant made the movement.

In order to know these M-values, each participant was also asked to report the exact moment he or she made the move. The time figures obtained by means of the variables V and M provided information about the difference in time that existed between the moment in which the person felt the desire to perform the movement and the exact moment in which the movement was performed.

To make the experiment much more reliable, Libet and his collaborators used a series of objective measurements or records. These consisted of the measurement of the preparation potential of the brain areas related to movement and an electromyography of the muscles involved in the specific activity that the participants were asked to perform.

Results of the experiment

The discoveries and conclusions made after the measurements were made and the study concluded left no one indifferent.

At first, and as expected, the participants of the study placed the variable V (will) before the variable M. This means that they perceived their conscious desire to perform the movement as previous to it. This fact is easily understood as a correlation between brain activity and the person’s subjective experience.

Now, the data that really made a revolution were those extracted from the objective records. According to these figures, the preparation brain potential appeared before the subject was aware that he wanted to move his wrist ; specifically between 300 and 500 milliseconds earlier. This can be interpreted as our brain knowing before ourselves that we want to perform an action or movement.

The conflict with free will

For Libet, these results conflicted with the traditional conception of free will. This term, typical of the field of philosophy, refers to the belief that the person has the power to freely choose his own decisions .

The reason was that the desire to perform a movement considered as free and voluntary is, in fact, preceded or anticipated by a series of electrical changes in the brain. Therefore, the process of determination or desire to perform a movement begins unconsciously.

However, for Libet the concept of free will continued to exist; since the individual still retained the conscious power to voluntarily and freely interrupt the movement.

Finally, these discoveries would imply a restriction to the traditional conception of how freedom and free will works, considering that the latter would not be in charge of initiating the movement but of controlling and ending it.

Criticisms of this research

The scientific-philosophical debates about whether people are really free to make decisions or whether, on the contrary, we are subject to a materialistic biological determinism , go back many centuries before Libet’s experiment and, of course, still continue today. So, as expected, Libet’s experiment was not free from criticism neither by philosophy nor by neuroscience.

One of the main criticisms made by some thinkers of the theories of free will is that, according to them, the existence of this brain preview should not be incompatible with this belief or concept. This brain potential could be a series of automatisms linked to a state of passivity of the person. For them, Libet would not be focusing on what is really important, the most complicated or complex acts or decisions which require previous reflection.

On the other hand, with regard to the evaluation of the procedures carried out in the experiment, the methods of counting and measuring time have been questioned, since they do not take into account how long it takes for the different brain areas to send and receive the messages.